The field of the present invention relates to infectious bronchitis virus (IBV) and methods for passaging IBV. The disclosed methods may be utilized to prepare vaccine compositions comprising the passaged IBV.
In the poultry industry avian infectious bronchitis (IB) coronavirus (IBV) continues to be the most common contributor to respiratory disease in chicken populations despite worldwide extensive vaccination with a multiplicity of type-specific vaccines. IBV replicates primarily in the upper respiratory tract causing respiratory disease in large chicken populations. IBV's surface (S) glycoprotein is post-translationally cleaved into a S1 subunit (˜550 amino acids) and a S2 subunit (˜600 amino acids) (Lai and Holmes, 2001). Like other coronaviruses, the S1 subunit of the S glycoprotein is responsible for viral attachment to cells and is important for host protective immune responses as it induces virus neutralizing-antibodies (Cavanagh, 1981, 1983, 1984; Cavanagh and Davis, 1986; Koch et al., 1990; Koch and Kant, 1990; Mockett et al., 1984). Because of the relevance of S1 for the first step of replication (i.e., attachment to cells) and immunological escape, the extensive variation exhibited by the S1 glycoprotein among IBV coronaviruses (Kusters et al., 1987; Kusters et al., 1989b) is likely the most relevant phenotypic characteristic for this virus's “adaptation” and evolutionary success (Toro et al., 2012b). Genetic diversity among coronaviruses is achieved by high mutation frequency and recombination events (Enjuanes et al., 2000a; Enjuanes et al., 2000b; Lai and Cavanagh, 1997; Stadler et al., 2003). Selection acting on diverse populations results in rapid evolution of the virus and the emergence of antigenically different strains (Toro et al., 2012b). More than 30 different IBV types have been identified during the last 5 decades in the U.S. alone. According to a 2012 review, over 50 different genotypes of IBV are currently affecting chicken populations worldwide (Jackwood, 2012). Multiple recent surveillance studies performed in the U.S. have demonstrated that serotypes/genotypes Arkansas (Ark), Massachusetts (Mass). Connecticut (Conn), DE072, Georgia variants GAV and GA98 are currently the most prevalent (Jackwood et al., 2005; Nix et al., 2000; Toro et al., 2006).
Because IBV exists as multiple different serotypes that do not provide for cross-protection after host exposure, a multiplicity of serotype-specific IBV vaccines have been developed worldwide. For example, vaccination programs in the U.S. currently comprise mono- or polyvalent vaccines including Mass. Conn., GA98, DE072, and Ark serotypes. In Europe, IBV vaccines commonly include strains belonging to serotypes UK4/91, D274, and D-1466. However. IBV's high ability to evolve allows it to consistently circulate in commercial poultry and cause outbreaks of disease in spite of extensive vaccination. In addition, accumulating evidence indicates that attenuated IBV vaccines may also be contributing to the emergence and circulation of vaccine-like viruses in host populations (Toro et al., 2012b; Toro et al., 2012c). Indeed, viral sub-populations differing from the predominant live vaccine population have been shown to emerge during a single passage of attenuated IBV vaccine in chickens (McKinley et al., 2008; van Santen and Toro, 2008).
In an effort to understand the mechanisms underlying the emergence of vaccine-like viruses, S1 gene sequences of virus populations of all four commercially available IBV Ark-serotype attenuated vaccines were analyzed before and after replication in chickens (Gallardo et al., 2010; van Santen and Toro, 2008). The results from these analyses demonstrated different degrees of genetic heterogeneity among Ark-derived vaccines prior to inoculation into chickens, ranging from no apparent heterogeneity to heterogeneity in 20 positions in the S gene. In all except one position, nucleotide differences resulted in different amino acids encoded and therefore in phenotypic differences among subpopulations present in the vaccines. Significantly, it has been observed that specific minor subpopulations present in each of the vaccines were rapidly “selected” during a single passage in chickens. Indeed, by 3-days post-ocular vaccination, viral populations with S gene sequences distinct from the vaccine major consensus sequence at 5 to 11 codons were found to predominate in chickens (Gallardo et al., 2010; McKinley et al., 2008; van Santen and Toro, 2008). Thus, the use of attenuated coronavirus vaccines may be contributing to the problem of antigenic variation, and the development of a novel vaccine technology to increase the resistance of chicken populations to IBV and reduce economic losses is essential for the poultry industry.